This invention relates to infrared photodiode array detectors and, more particularly, to a mercury-cadmium-telluride photodiode array detector having a plurality of mesas etched into layers of compositionally graded material, and an overlying layer of surface passivation which has a fixed positive charge.
An infrared photodiode array detector is used to convert incident infrared radiation to a detectable electrical current. Typically such an array is composed of a number of individual photodiodes arranged in a linear or two dimensional matrix. The magnitude of the current generated by each of the diodes within the array is in direct relation to the flux density of the incident infrared radiation impinging upon each of the diodes.
HgCdTe photodiode arrays are typically composed of a mercury-cadmium-telluride base layer which is uniformly doped with a substance suitable to give the base layer the characteristics of a p-type semiconductor material. Within a surface of this p-type base layer are then formed a number of n-type semiconductor regions. The interface of each n-type region with the p-type base layer results in the formation of a p-n diode junction. A current is caused to flow through each diode junction by charged photocarriers which are produced when the base layer absorbs incident infrared radiation. These photocarriers diffuse to the diode p-n junction, resulting in a diode current.
A particular problem with this type of photodiode array is that the surface of the base layer surrounding the p-n junction can create a diode leakage current. This leakage current flows even when no radiation is incident upon the array, resulting in what is commonly called dark current. The dark current adversely affects the signal-to-noise ratio of the photodiode array.
Some prior photodiode arrays, in an attempt to reduce the leakage current, utilize a field plate to control the base layer surface potential. However, this approach creates a problem in that a metalization procedure is required to fabricate the field plate. The metalization procedure results in an array which is more difficult and costly to manufacture. The use of a field plate also has the disadvantage of requiring an external voltage source to properly bias the plate.
Another problem with previous HgCdTe arrays arises from the inherent compositional uniformity of the base layer material. Because the ratio of mercury to cadmium is essentially constant throughout the base layer, and the entire base layer is of one energy bandgap and hence, capable of absorbing radiation. Thus charged photocarriers can be created anywhere within the base layer. It can be seen, therefore, that photocarriers are free to travel within the base layer in any given direction. Thus, a photocarrier which is created between two neighboring diodes is free to travel to either, resulting in the problem known generally as crosstalk between diodes. Excessive crosstalk can detrimentally affect the signal-to-noise ratio of the array, with a consequent impairment of the clarity of the image as viewed by the array.
In order to compensate for the reduction in the signal-to-noise ratio caused by surface leakage and crosstalk, previous photodiode arrays often utilize higher impurity doping levels. This is done in an attempt to reduce surface leakage effects and minimize crosstalk.
The use of higher doping levels, however, creates an additional problem in that the useable lifetime of the semiconductor material is reduced, which may cause a reduction in the conversion efficiency of the array. In addition, leakage current across the diodes within the array may increase, resulting in an increase in the dark current. A higher doping level may also cause a detrimental increase in the junction capacitance of each diode, with a corresponding decrease in the signal-to-noise ratio of the photodiode array when coupled to a readout device.